Night vision systems allow a vehicle occupant to better see objects during relatively low visible light level conditions, such as at nighttime. Night vision systems typically are classified as either passive night vision systems or active night vision systems. Passive systems simply detect ambient infrared light emitted from objects within a particular environment. Active systems utilize a light source to illuminate a target area and subsequently detect infrared light reflected off objects within that area.
Passive systems typically use far-infrared cameras characterized by low resolution and a relatively narrow field-of-view. Such cameras must be located on the vehicle exterior in order to acquire requisite infrared energy in the operating environment. Externally mounted cameras can negatively affect vehicle styling. Far-infrared cameras are also costly to manufacture and generate images that have poor contrast, which can be difficult to interpret.
Active systems provide improved resolution and image clarity over passive systems. Active systems utilize laser or incandescent light sources to generate an illumination beam having near infrared light energy, and charged coupled devices (CCD) or complementary-metal-oxide-semiconductor (CMOS) cameras to detect reflected infrared light. Active systems commonly deploy a light source external to the vehicle so as to transmit a significant amount of light energy and provide a bright scene for imaging.
Diode lasers are preferred over incandescent light sources for several reasons. Incandescent light sources are not monochromatic like diode lasers, but instead emit energy across a large spectrum, which must be filtered to prevent glare onto oncoming vehicles. Filtering a significant portion of energy from a bulb is expensive, energy inefficient, and generates undesired thermal energy. Also, filter positioning is limited in incandescent applications, since the filter must be located proximate an associated light source. As well, multiple incandescent sources are often required to provide requisite illumination, thus increasing complexity and costs.
Exterior mounted light sources or cameras are undesirable due to risk of damage during a vehicle collision. Night vision components are relatively expensive and, as a result, protection of the components is desired. Also, exterior mounted light sources and cameras are susceptible to theft. Additionally, external mounting of sources and cameras can limit and compromise vehicle design and styling, can be esthetically displeasing, and can increase exposure of the devices to dust and debris. Exposure to dust and debris negatively effects performance of the sources and cameras. When the sources and cameras are dirty, light transmission and reception can be substantially reduced, and compromise system performance.
Exterior mounted semiconductor illumination sources have additional associated disadvantages. A significant disadvantage is controlling the wavelength of the illumination beam. Night vision systems have a preferred wavelength operating range. When a night vision system is operated outside this range the received illumination decreases, negatively affecting image quality of the night vision system. The diode laser emission wavelength is sensitive to change in temperature, such that the wavelength of a diode laser output shifts approximately 0.25 nm for every one-degree Celsius temperature change. Since external temperatures vary considerably, it is difficult to control the temperature of a diode laser. Also, when mounted externally, a risk of exposure to water exists, which can render the laser inoperable. Sealing and housing problems due to thermal energy management may also arise when weatherproofing diode lasers.
Furthermore, in designing a vehicle exterior, the external light source may have to be customized to satisfy styling requirements. Thus it is difficult to achieve commonality for light sources between different vehicles. Designing different light sources for different vehicles is costly.
In the parent application, U.S. patent application Ser. No. 10/604,376, a bandpass filter is used to detect a light reflected from objects in the scene. The bandpass filter is used to remove most of the light from oncoming headlights which prevents the camera image from blooming and enables the system to see into the lights of the opposing vehicles. The system uses a filter having a full-width-at-half-maximum (FWHM) of about 13 nm. This value is relatively high and still allows a substantial amount of light from oncoming headlights to be introduced into the system. The value of the filter is chosen to accommodate the manufacturing tolerances between the various light sources. It would therefore be desirable to provide an improved active night vision system that reduces the filter FWHM value substantially below that of 13 nm to allow even more light to be filtered from oncoming headlights.